1. Field of the Invention
The present invention relates to an image signal processor which generates an image signal based on various color signals obtained by an image pick-up device in which pixels of each color are arrayed in a mosaic pattern.
2. Description of the Related Art
It is possible to obtain a color image through one solid image pick-up device such as a CCD (Charge Coupled device) image sensor by arraying a plurality of colors of color filters in a mosaic pattern over a light reception area of the device. One example of the color filter mosaic array is a Bayer array shown in FIG. 1. This array comprises, for example, filters of three primary colors of red (R), green (G), and blue (B), in which a unitary block consisting of four pixels (2×2) is arranged repeatedly in both row and column directions. Specifically, in each odd-numbered line the G and B colors are arrayed alternately and in each even-numbered line the R and G colors are arrayed alternately, thus giving two G-color pixels and respective one R-color and one B-color pixels in each block.
The image pick-up device gives color signals, that is, image signals each of which corresponds to each color. To display an image, it is necessary to have a luminance signal, that is, an image signal obtained by conducting white-balance compensation on these color signals and then mixing them at a predetermined ratio.
Furthermore, a CCD camera which uses a single CCD image sensor incorporates an optical LPF (Low-Pass Filter) between the CCD image sensor and an image pick-up lens to avoid fold-back distortion caused by sampling. This filter cancels those high-frequency components of an optical image that are made incident upon the CCD image sensor to have at least half a horizontal sampling frequency fH.
If a luminance signal is generated on the basis of color signals obtained by an image pick-up device in which such filters of different colors are arrayed alternately, moire noise occurs in the luminance signal corresponding to an arraying interval of the filters. To cancel the moire noise, the luminance signal must be filtered through a trap filter, which minimizes response at half the horizontal sampling frequency fH.
The conventional technology is effective to suppress moire noise in a luminance signal obtained, for example, when a single-color subject is photographed in a state in which a level balance of the colors is sufficiently adjusted. However, if, for example, the color differs greatly across a boundary line as in a case where a red object exists on a white background, a kind of moire noise called jagged pixellization is liable to occur at the boundary. The jagged pixellization becomes more significant as the attenuation characteristics becomes more steep near fH/2 of the trap filter. Conversely, by moderating the steepness of the trap filter characteristics, that is, by expanding a frequency band width in which the signal is attenuated to thereby suppress response near fH/2, the moire noise can be reduced, including the jagged pixellization. This method, however, has a problem that the resolution of the luminance signal is compromised after it is filtered.